The present invention relates generally to magnetooptic storage apparatus and methods, and more particularly to apparatus and methods which provide for direct overwriting of magnetooptic recording media.
Erasable optical memory has the potential of replacing magnetic recording devices in many mass data storage applications. One of the main factors preventing this is the inability of current systems to directly overwrite the recording medium at high data rates. Currently, there are several approaches that are under investigation to overcome the overwriting limitation. These primarily include magnetic field modulation, and a dual magnetic layer medium. Other concepts have been proposed, but are complex media structures which are theoretical in nature or whose characteristics are not well-suited for data recording.
The magnetic field modulation scheme involves the use of a magnetic head in close proximity to the active layers of the magnetooptic medium. The data rate of such a system is limited by the switching characteristics of the coil in the magnetic head. To achieve high rates, the head must be small and be positioned in extremely close proximity to the active layers which creates alignment and reliability problems. This technique is believed to be limited to data rates below 2.5 million bytes per second by practical considerations. The dual magnetic layer medium has been demonstrated, but such structures are difficult to manufacture due to the sensitivity of interface between the active layers to small changes in impurity content at the interface and manufacturing conditions.
Direct overwriting of magnetooptical materials is presented in detail in the following articles and patents: "The Overwriting Characteristics of Magneto-Optical Disk by Magnetic field Modulation Method," by Fujio Tanaka et al, in IEEE Transactions on Magnetics, Vol. 23, pages 2695-2698 (1987); "High-Speed Overwritable Optical Disk," by Masahiro Ojima et al, in SPIE Proceedings, Vol. 899, pages 154-160 (1988); "Magneto-Optic Recording Erase Method," published in the IBM Technical Disclosure Bulletin, Vol. 29, No. 5, page 2093, October 1986; U.S. Pat. No. 4,649,519 entitled "Self Biasing Thermal Magneto-optic Medium"; "Operating Margins for Magneto-Optic Recording Materials with Direct Overwrite Capability," by Han-Ping D. Shieh et al, IEEE Transactions on Magnetics, Vol. MAG-23, Vol. 1, pages 171-173, January 1987; and "Magneto-optic recording materials with direct overwrite capability," by Han-Ping D. Shieh et al, Appl. Phys. Lett. Vol. 49, No. 8, pages 473-474, Aug. 25, 1986.
With reference to the Tanaka et al article, at page 1, column 1, in the third paragraph of the introduction thereof, reference is made to two articles. The first article is entitled "Erasable Magneto-Optical Recording Media," by M. Hartmann et al, in IEEE Transactions on Magnetics, Vol. MAG-21, (1985) at page 1013. The second article is authored by K. Torazawa, et al and may be found at page 231 of the "Digest of Symposium on Optical Memory Japan," (1986) at page 87. With reference to the Hartmann et al article, at page 1017 it discloses a technique of directly overwriting an optical disk. This technique involves the use of a large area magnet whose field overlays a wide area of the optical disk. A first laser beam is employed to erase the optical disk on a first track thereof, while a second laser beam is employed to write to the erased optical disk on an adjacent track of the disk. When writing of the second track is finished, the direction of the magnetic field is switched, the second laser beam is moved to the previously erased track and the first beam is moved to the next track. This technique uses global magnetic field, and the field must be flipped for successive erasing and writing operations. This technique is useful for erasing and writing long files, but does not permit direct overwriting of a random single sector of data.